Found on: (7) Twilight items:
Effect: This suit of armor favored by eladrin spellcasters, becomes semi-incorporeal when donned. The twilight armor reduces the chance of arcane spell failure by -20%.
Found on: (4) Greater Twilight items:
Note: These enchantments reduce only the spell failure of the items they are on. I.e., a Greater Twilight armor won't reduce the spell failure caused by your shield. Other effects, such as Arcane Casting Dexterity or Blue Augment can affect your total spell failure.